Does resizing a vector invalidate iterators?

I found that this C++ code:

vector<int> a;
a.push_back(1);
a.push_back(2);
vector<int>::iterator it = a.begin();
a.push_back(4);
cout << *it;

print some big random number; but if you add a.push_back(3) between 3rd and 4th lines, it will print 1. Can you explain it to me?